1. Field of the Invention
The present invention relates to a optical communications apparatus and optical communications system which are used to generate sequences of optical signals. For example, the present invention is used in optical communications networks using an RZ (return to zero) transmission system.
2. Description of Related Art
Conventionally, a system using optical solitons as pulse signal light has been known as one type of optical communications system based on RZ transmission. The term “optical solitons” refers to stable optical pulses that are propagated through optical fibers in a state in which the spread of these optical pulses is suppressed by a self phase modulation effect.
Furthermore, the dispersion-managed soliton transmission system is known as one type of RZ transmission system using optical solitons. By using this dispersion-managed soliton transmission system, it is possible to reduce the rate of signal error generation in long-distance transmission by further suppressing waveform distortion of the optical solitons. For example, this dispersion-managed soliton transmission technique is disclosed in the following reference:
“Dispersion-manage soliton interactions in optical fibers”, T. Yu et al., OPTICS LETTERS, Vol. 22, No. 11, p. 793, 1997.
In dispersion-managed soliton transmission, a transmission path is constructed by combining a plurality of different types of optical fibers with different optical characteristics; furthermore, pulse signal light with an appropriate light intensity and pre-chirp is used. Dispersion-managed soliton transmission makes it possible to balance the wavelength dispersion and nonlinear effects of the optical fibers; accordingly, waveform distortion of the pulse signal light can be suppressed.
The simplest dispersion-managed soliton transmission path is constructed from a fiber that has a positive wavelength dispersion (anomalous-dispersion fiber) and a fiber that has a negative wavelength dispersion (normal-dispersion fiber) (see the abovementioned reference, p. 793, line 12 to line 17). Specifically, this dispersion-managed soliton transmission path compensates for the wavelength dispersion of the transmission path by combining an anomalous-dispersion fiber and a normal-dispersion fiber.
One of the causes of degradation of dispersion-managed soliton transmission is nonlinear interaction between pulses (i. e., soliton interaction). This soliton interaction consists of attractive and repulsive forces that act between the optical pulses (i. e., optical solitons) that are propagated through the optical fibers. When such soliton interaction acts on the optical solitons, a time shift (time jitter) is generated between adjacent optical solitons. Furthermore, in cases where this soliton interaction is extremely large, the adjacent solitons collide with each other. This distance from the starting point of transmission to the point of collision is called the collision distance or interaction length, and is an important indicator that expresses the properties of optical solitons. The collision distance depends on the dispersion management intensity, optical pulse waveform and the like.
The dispersion management intensity is also called the map intensity, and is generally expressed as a parameter γ (see FIG. 1 in the abovementioned reference). This parameter γ varies according to the types of fibers that make up the dispersion-managed soliton transmission path. As is shown in FIG. 3 of the abovementioned reference, the collision distance reaches a maximum length when this parameter γ is approximately 3.3.
Furthermore, as is shown in FIG. 3 of the abovementioned reference, the collision distance increases with an increase in the ratio τS/τ0 of the pulse interval τS to the pulse width (half-value width) τ0 of the optical pulses formed by the optical solitons (i. e., with a decrease in the pulse width τ0).
Accordingly, by setting the map intensity parameter γ at 3.3 and setting τS/τ0 at as large a value as possible, it is possible to reduce the effects of soliton interaction so that good dispersion-managed soliton transmission over a long distance becomes possible.
In single-channel dispersion-managed soliton transmission, the quality of long-distance transmission can be improved by reducing the effects of soliton interaction as described above.
However, in the case of fine wavelength-division multiplex dispersion-managed soliton transmission, nonlinear interaction between channels, i. e., cross phase modulation (XPM), appears to a conspicuous degree in addition to nonlinear interaction between pulses, i. e., soliton interaction.
In the case of fine wavelength-division multiplex transmission, the wavelength varies from channel to channel; accordingly, the optical solitons of different channels collide with each other and pass each other during propagation through the optical fibers. In the case of such collisions, the optical solitons act as perturbations with respect to the other optical solitons. This action is cross phase modulation. Cross phase modulation causes the center frequencies of the signals formed by the optical solitons to be displaced, and causes waveform distortion of the optical solitons.
The signals formed by the optical solitons have various code patterns according to the information being transmitted. Accordingly, when optical solitons collide with a plurality of optical solitons of different channels during propagation through the optical fibers, various frequency variations and waveform distortions of the optical solitons result. These frequency variations and waveform distortions cause the generation of time jitter and intensity jitter.
Cross phase modulation reduces as the ratio Δω/ω0 of the frequency interval Δω of the adjacent channels to the width ω0 of the frequency spectrum of the optical signals increases. Accordingly, in a case where the frequency interval Δω is fixed, the effects of cross phase modulation diminish with a decrease in the width ω0 of the frequency spectrum. However, when the width ω0 of the frequency spectrum is decreased, the pulse width τ0 of the optical pulses increased. As was described above the collision distance between optical solitons within the same channel becomes shorter as the ratio τS/τ0 of the pulse interval τS to the pulse width τ0 decreases; accordingly, the quality of long-distance transmission is lost.